strange noise when idle rpms drop after getting tune

[tombo9876]

I got a tune awhile ago, but never got around to posting about this. The car runs great. I'm just curious more than anything. If I start the car cold, it'll go like normal for a few seconds, and then it sounds like something engages, and the RPMs drop, and sound almost like a hum comes on. I've included a video here. The car still operates fine, just curious as to what it is. It never did this before the tune

Idle RPM drop - YouTube

 

[JDUB348]

Maybe its just where the idle was "set" in the tune :shrug:

 

[10sec89]

Maybe alternator turning on and causing a drag almost like the A/C compressor.

 

[Lobo_Loco]

Maybe alternator turning on and causing a drag almost like the A/C compressor.

yep im with you

 

[tombo9876]

I wasn't aware than an alternator "turned on" I thought spinning = its on. And this only started happening with the tune. Just curious as to what it is. The sound is what is new to me, not the fact that the idle drops

 

[10sec89]

Alternators do switch on and off causing drag on the engine. I suspect yours might be going out.

 

[LordVader04]

My turner did a fuel cut off when I am off the gas pedal. I had no idea until I asked him about it, sometimes it makes gargling noises on deceleration, maybe that is it.

 

[tombo9876]

So, I finally did a data log to try to figure it out. Nothing really jumped out, but there are a few things to note:

When the rpms drop, and the sound starts, the following happen:

• FPDC stays the same
• Load jumps from 0.23 to 0.37, then falls back down to 0.32
• Air flow jumps from 1500 to 2250, then falls down to 1800
• STFTs, both banks, go from ~0.76 to ~0.72, then rise back up to around 0.76
• Spark goes from 37.5 to 7.5, and stays there. Related to this, spark source
  starts at 2, goes to 8 momentarily, then goes to 9 and stays there, right around the same time
• Battery voltage does NOT change which I think rules out the alternator thing

So, nothing helpful, but I would love to figure out the sound. Maybe before the tune, the idle drop was more gradual, so it's just a lot more noticable now?? I do not know